System and method for mitigating interference in pulse oximetry

ABSTRACT

A pulse oximetry sensor adapted to emit light from an emitter proximate to a patient&#39;s tissue and detect a portion of the emitted light on a detector proximate to the tissue. The pulse oximetry system is adapted to acquire position data for the emitter and for the detector with one or more position indicators. The pulse oximetry system is adapted to process the position data to obtain a change in position of the emitter and the detector; and to process pulse oximetry measurements using the change in position to obtain a motion-corrected pulse oximetry data.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No. 11/529,081, filed Sep. 28, 2006 to Carine Hoarau, the disclosure of which is hereby incorporated by reference in its entirety for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to pulse oximetry and, more particularly, to mitigation of interference in pulse oximetry.

2. Description of the Related Art

This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.

In the field of medicine, doctors often desire to monitor certain physiological characteristics of their patients. Accordingly, a wide variety of devices have been developed for monitoring physiological characteristics of a patient. Such devices provide doctors and other healthcare personnel with the information they need to provide the best possible healthcare for their patients. As a result, such monitoring devices have become an indispensable part of modern medicine.

One technique for monitoring certain physiological characteristics of a patient is commonly referred to as pulse oximetry, and the devices built based upon pulse oximetry techniques are commonly referred to as pulse oximeters. Pulse oximetry may be used to measure various blood flow characteristics, such as the blood-oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and/or the rate of blood pulsations corresponding to each heartbeat of a patient. In fact, the “pulse” in pulse oximetry refers to the time varying amount of arterial blood in the tissue during each cardiac cycle.

Pulse oximeters typically utilize a non-invasive sensor that transmits light through a patient's tissue and that photoelectrically senses the absorption and/or scattering of the transmitted light in such tissue. One or more of the above physiological characteristics may then be calculated based upon the amount of light absorbed or scattered. More specifically, the light passed through the tissue is typically selected to be of one or more wavelengths that may be absorbed or scattered by the blood in an amount correlative to the amount of the blood constituent present in the blood. The amount of light absorbed and/or scattered may then be used to estimate the amount of blood constituent in the tissue using various algorithms. Changes in the amount of arterial blood in the tissue during a blood pressure pulse may change the amount and character of the light detected by the sensor's photodetector.

Normally, obtaining pulse oximetry measurements involves physically attaching a sensor to an anatomical part, whereby the attachment can be accomplished in various ways, depending on the type of sensor and the anatomical part in question. Accordingly, this attachment can substantially influence the quality of the pulse oximetry measurement, which depends on the sensor's ability to detect changes in the concentration of arterial blood relative to other tissue structures in the portion of the tissue illuminated by the sensor. Therefore, motion of the sensor relative to the tissue or changes in tissue during a pulse oximetry measurement, such as voluntary or involuntary movements can lead to changes in the spatial relationship between the sensor and the tissue. Consequently, the light's optical path can change, which may cause the light emitted by the sensor to interact with different tissue structures and tissue surfaces having different levels of blood perfusion and/or different absorption scattering characteristics. Thus, the motion of the sensor relative to the tissue can result in variations of light intensities detected by the sensor during the measurement process, adversely affecting the values of physiological parameters derived from a pulse oximetry measurement. Such related-variations and aberrations within the derived data are typically referred to as interference. Unfortunately, such interference may give a false indication on the state of the physiological parameter being measured, and thus, degrade the accuracy and reliability of the physiological parameter obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the invention may become apparent upon reading the following detailed description and upon reference to the drawings in which:

FIG. 1 illustrates a block diagram of a system configured to generate motion-corrected pulse oximetry parameters in accordance with one aspect of the present technique;

FIGS. 2A, 2B, and 2C, illustrate a cut-away side view of an exemplary clip-style pulse oximetry sensor on a patient's finger in accordance with one aspect of the present technique;

FIGS. 3A, B and C illustrate a cross-sectional view of a pulse oximetry sensor enclosing a patient's digit in accordance with one aspect of the present technique;

FIGS. 4 and 5 illustrate a top view of a bandage-style pulse oximetry sensor on a patient's finger in accordance with one aspect of the present technique; and

FIG. 6. illustrates a pulse oximetry system coupled to a multi-parameter patient monitor in accordance with one aspect of the present technique.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.

Accordingly, the following technique describes a system and method for mitigating interference in pulse oximetry and to thereby increase the accuracy and reliability of any physiological parameters derived form the pulse oximetry measurement thereof. Specifically, it is desirable to have at least one position indicator that can measure the positions of one or more reference points in space and time, such as for example, the orientation of the emitter with respect to the detector at any point in time. Moreover, the data acquired by the position indicator should be processed synchronously with the pulse oximetry data to reduce noise in the pulse oximetry data or other physiological parameters.

Referring to FIG. 1, a technique is depicted for acquiring and using positional and pulse oximetry data to mitigate interference. In particular, block diagram 10 illustrates an embodiment of the present technique utilizing time correlated acquisition and processing of pulse oximetry and position data. As such, embodiment 10 depicts the manner in which initial uncorrected pulse oximetry data is corrected to yield pulse oximetry physiological parameters having less noise and interference.

System 10 comprises a sensor 12 that acquires pulse oximetry data 18. A position indicator 14 acquires position data 16 of the emitter and the detector of the pulse oximetry sensor 12. A processing unit 20, such as a CPU or microprocessor typically found on a general or special purpose computer or patient monitor, acquires and correlates the pulse oximetry data 18 and the position data 16 for further processing.

The time correlated position data 24 is processed, such as by a processing unit 26, to generate coordinate data 28 which describes the position of the emitter and detector in space and time. Coordinate data 28 is analyzed and compared over time, such as by processing unit 30, to determine changes in the position of the emitter and/or detector over a certain interval of time. As will be appreciated by those of ordinary skill in the art, the various processing units and comparators 20, 26, 30 and 34 discussed herein may in practice be one or more CPU's, microprocessors or dedicated processing circuits located on a general or special purpose computer or patient monitor.

The changes in the position of the emitter and the detector when identified may be used to calculate correction factors/parameters 32, such as, the optical path length between the emitter and the detector of the sensor 12. Correspondingly, processing unit 34 receives corrections factors/parameters 32 from processing unit 30 along with time-correlated pulse oximetry data 22 and calculates corrected pulse oximetry data 36 based on the position of the emitter and detector during acquisition of the respective pulse oximetry data 18. In the process of doing so, processing unit 34 may employ various algorithms and data banks when it corrects the data 22 for interference.

The processing unit 34 may employ a data bank of optical path lengths as a function of light scatter/absorbance probabilities. Thus, for example, patient motion leading to an increase in the optical path length between light emitting and receiving components of the sensor 12 can lead to an increase of light absorption in the tissue. This would be indicated in data 18 as a decrease in the amount of light received by the sensor 12. Accordingly, upon detecting a change in the optical path length, processing unit 34 would correct for the excess absorbance by utilizing absorbance probabilities/optical path length functions. Hence, interference is eliminated by combining time-correlated pulse oximetry data 22 with correction data 32 to yield corrected pulse oximetry data 36. Lastly, the corrected data 36 may be transferred to a display 40 for review by a health care provider.

A pulse oximetry sensor which detects motion and generates the pulse oximetry data 18 and the position data 16 for use in system 10 is depicted in FIGS. 2A-2C. Accordingly, these figures illustrate an exemplary clip-style pulse oximetry sensor typically placed on a patient in a location, such as a digit, that is normally perfused with arterial blood to facilitate proper light absorption. However, other common sensor sites include a patient's toes or earlobes. Pulse oximetry sensors used on these sensor sites are typically “transmission type” sensors. Transmission type sensors include an emitter and detector that are typically placed on opposing sides of the sensor site. If the sensor site is a fingertip, for example, the cuff, clip, or bandage associated with the pulse oximetry sensor is positioned over the patient's fingertip such that the emitter and detector lie on either side of the patient's nail bed. In other words, the sensor is positioned so that the emitter is located on the patient's fingernail and the detector is located 180° opposite the emitter on the patient's finger pad. During operation, the emitter shines one or more wavelengths of light through the patient's fingertip, and the light received by the detector is processed to determine various physiological characteristics of the patient. For determining the oxygen saturation of the patient's arterial blood, two or more wavelengths are used, most commonly red and near infrared wavelengths.

Accordingly, FIGS. 2A-2C illustrate a sensor 50 and different postures a digit 52 may attain relative to the sensor 50, as a result of patient motion. As such, variations of the digit's posture may lead to variations in the amount of light scattered or absorbed between emitter and detector of the sensor 50. For example, FIG. 2A illustrates a digit in a neutral posture relative to the sensor 50, such that the clip-portion of the sensor 50 is open in a regular width. However, motion of the digit 52, as illustrated in FIG. 2B, may result in a flexed digit-sensor posture which may lead to a wider opening of the clip. Consequently, this may influence the amount of light that reaches the detector. Similarly, a patient pressing the finger against the clip while the clip is situated on a hard surface, may lead to a narrower opening of the clip, as illustrated in FIG. 2C. This, too, may influence the amount light scattered/absorbed by the tissue.

More specifically, sensor 50 of FIGS. 2A-2C is configured to provide position data 16 (FIG. 1) for the emitter 56 and detector 57, thereby allowing motion correction of pulse oximetry data 18 (FIG. 1) acquired by the sensor 50. Sensor 50 comprises a sensor body that includes the emitter 56, detector 57, and one or more position indicators 58 and 60 proximate to at least one of the emitter 56 and or detector 57. In the configuration shown, the finger clip comprises rigid bottom and top portions 62 and 64 respectively. Accordingly, as the clip opens or closes, the bottom and top portions 62 and 64 are free to move on a vertical plane.

Mounted on structures 62 and 64 are emitter 56 and detector 57 respectively. Similarly, mounted on structures 62 and 64 are position indicators 58 and 60 respectively. The position indicators 58 and 60 are disposed laterally to the emitter 56 and detector 57, respectively, and are separated from them by a known, fixed distance.

The separation distance between the center of each of the portions 62 and 64 and the center of clip is denoted by a line 66. Similarly, the separation distance between position indicators 58 and 60 is denoted by line 68, determined by the opening of the clip. Hence, the space coordinates of the emitter 56 and the detector 57 are obtainable once the separation distances 66 and 68 are known.

Determining the separation distance 64 can be done in various ways. An exemplary method would be to adapt the position indicators 58 and 60 as contact points of a linear resistor embedded in a linear potentiometer device. Accordingly, in a configuration where the digit 52 acts as a variable linear resistor while it is placed between the two indicators 58 and 60, a voltage across the finger can be used to correlate to the distance between the contact points 58 and 60. Hence, the voltage across the digit 52 is directly correlated to the optical path length between the emitter 56 and the detector 57. Thus, motion of the digit 52 relative to the sensor body may cause a change in the position of the digit 52 relative to the emitter 56 and/or the detector 57. Correspondingly, a change in the distance between the contacts 58 and 60 is anticipated as well, as illustrated in FIGS. 2B and 2C. Thus, the displayed configuration establishes correlations between measured potentiometer voltages and digit dislocations. Accordingly, a suitable algorithm can be employed in which changes in measured potentiometer voltages can used to the correct interference in pulse, as discussed with reference to FIG. 1. As will be appreciated by those of ordinary skill in the art, other types of potentiometers may also be similarly employed. For example, potentiometers that measure bending or flex of a substrate by changes in resistance may be employed in place of the linear potentiometer described above to derive the same or analogous spatial data. An example of such a potentiometer is the Bend Sensor® available from Flexpoint®.

Alternatively, employing magnetic field variation sensing using devices, such as Hall Effect Devices, Magneto-Resistive Devices, magnets, and the like are means for providing spatial data for mitigating interference. Applying such means in pulse oximetry may, for example, be useful in determining parameters, such as the optical path length between the emitter and the detector of a pulse oximetry sensor.

Accordingly, exemplary embodiments of the present technique which utilize magnetic field variation sensing are depicted in FIGS. 3A-3C. An embodiment in accordance with the present technique shown by sensor 80 illustrates a sensor body 82 that completely or partially encloses a patient's digit 52. On the inner surface of the enclosure are an emitter 84 and a detector 86, both pointing upward and toward the digit 60. Accordingly, in this configuration the pulse oximeter sensor 80 is adapted to work in reflectance mode. Reflectance type sensors generally operate under the same general principles as transmittance type sensors. However, reflectance type sensors include an emitter and detector that are typically placed on the same side of the sensor site. For example, sensor 82 depicts an emitter 84 latterly disposed to a detector 86. Photons emitted by the emitter 84 are scattered and reflected off the digit 52 to the detector 86.

Furthermore, embedded inside the enclosure 82 are position sensors 90-94. Hence, the embodiment shown displays a position marker 88, such as a magnet, disposed on the patient's digit 52. Further, the three position sensor 90-94 may be configured as Hall Effect Devices disposed perpendicularly to each other. Furthermore, each position sensor is disposed from the emitter 84 and the detector 86 by a fixed distance. Thus, position marker 88 disposed on the patient's finger may provide a reference point to each of the three position sensors 90-94. Consequently, the three position sensors 90-94 may provide a three dimensional coordinate of the position marker 88 using, for example, a triangulation method.

Accordingly, during pulse oximetry data acquisition, motion of the patient's finger relative to the enclosure 82 brings about a change in position of the position marker 88 with respect to the position sensors 90-94. As such, sensor 80 can acquire data representing the coordinates of the patient's digit 52 relative to the emitter 84 and/or the detector 86.

In a further embodiment shown in FIG. 3B, sensor 80 is configured to determine a separation distance, denoted by reference numeral 85, between the emitter 84 and the detector 86 independent of their separation from the digit 52. Thus, an exemplary configuration may include, position sensor 90 comprising a Hall Effect device, and a position marker 91 comprising a magnet, disposed directly beneath the emitter 84 and the detector 86 respectively. Such a configuration may track the separation between the emitter 84 and the detector 86 as the digit 52 moves.

The foregoing configuration may also be useful in an exemplary embodiment shown in FIG. 3C, where the sensor body 82 of sensor 81 is a flexible enclosure fitted over a patient's flexed finger 52. Accordingly, motion of the digit 52 can cause a deformation of the sensor body, as it stretches and squeezes in various forms. The deformation may bring about a change in the distance between the emitter 84 and the detector 86, as denoted by distance 87. Accordingly, in a flexed posture, distance 87 may be greater or less than distance 85 (FIG. 3B). Hence the difference between the distances 85 and 87 may correspond to a change of the light's optical path as the digit 52 changes its posture. Thus, as the Hall Effect Device disposed in position sensor 90 moves relative to the magnet disposed in position marker 91 during the deformation of the sensor body, a voltage is generated across the Hall Effect device. This can be used to extrapolate the displacement of the emitter 84 relative to the detector 86. Using the position data generated by the relative motion of position sensor 90 and position marker 91, a suitable algorithm may be employed for mitigating pulse oximetry interference, as discussed with reference to FIG. 1.

Further, an embodiment of a remote positioning system including a pulse oximetry sensor and one or more position sensors is illustrated in FIG. 4. A sensor 100 includes a sensor body 102 in the form of a flexible bandage placed on a patient's digit 52. An emitter 104 and a detector 106 working in transmission mode are disposed on the sensor body 102. Accordingly, finger motion may cause stretching and/or compressing of the bandage-style sensor 102, leading to relative motion between the emitter 104 and the detector 106. Such motion may influence physical measurements on which derived pulse oximetry parameters depend, such as, the optical path length between the emitter 104 and the detector 106. In this embodiment, a combination of local and remote components provides the desired position data. Such components may include active and/or passive devices, markers, and/or media capable of emitting, receiving, reflecting, and/or absorbing electromagnetic radiation.

Thus, in the exemplary embodiment of FIG. 4, tracking the position of the emitter 104 and the detector 106 may be achieved with position sensors 108 and 110 disposed locally and in close proximity to the emitter 104 and the detector 106. For example, the position sensors 108 and 106 may each comprise an RF/IR transmitter configured to transmit a distinct RF/IR frequency. Correspondingly, receivers 112-116 disposed remotely from the sensor body 102 may be configured to receive the distinct RF/IR signals transmitted by the transmitters 108 and 110. Furthermore, the receivers 112-116 may be placed in a manner wherein each of the receivers is disposed perpendicularly to the other two receivers. Signals emitted by the transmitters 108 and 110 can be traced by the receivers 112-116 for triangulating the respective positions of the transmitters 108 and 106.

In another exemplary embodiment, as shown by FIG. 5, a sensor 130 with the sensor body 102 includes local position markers 132 and 134 disposed in close proximity to the emitter 104 and the detector 106. Accordingly, the position markers 132 and 134 may be configured as RF/IR reflectors illuminated by transmitter 136. Accordingly, the transmitter 136 may be configured as an RF/IR transmitter capable of transmitting at least two distinct RF/IR frequencies. These frequencies may be received by the receivers 112-116 configured to receive each of the frequencies reflected by position markers 132 and 134. Hence, by triangulation, the configuration above allows to obtain the coordinates of the emitter 104 and the detector 106. Accordingly, this data can be used to correct for interference resulting from unavoidable bandage and/or finger motion as discussed with reference to FIG. 1.

As one skilled in the art would appreciate, position indicator devices for mitigating interference in pulse oximetry are not limited to the ones that have been described thus far. There exist a multitude of other devices that can accomplish the same task such as: accelerometers, fiber optic sensors, interferometers, piezoelectric crystals, pressure gauges, magneto-resistive sensors and more.

Regardless of type, the sensors 50, 80, 100, and 130 are typically adapted to be coupled directly to a pulse oximetry system 200 shown in FIG. 6. The system 200 includes a monitor 210, connected to a computer 216 via cable 218. The monitor 210 is also connected to a pulse oximetry sensor 50, 80, 100, 130 via cable 212. However, it should be appreciated that the cable 212 may be coupled to a transmission device (not shown) to facilitate wireless transmission between the sensor 50, 80, 100, 130 the monitor 210. The monitor 210 may be any suitable pulse oximeter, such as those available from Nellcor Puritan Bennett Inc. 

What is claimed is:
 1. A pulse oximetry system comprising: a sensor comprising: a sensor body; an emitter disposed on the sensor body; a detector disposed on the sensor body; and one or more position indicators that generate a change in voltage or resistance in response to changes in position of the emitter and the detector; and a monitor comprising: a processor configured to: receive a signal from the detector and a signal from the one or more position indicators; determine a corrected path length based on the signal from the one or more position indicators; and determine a physiological parameter based at least in part upon the signal from the detector and the corrected path length.
 2. The pulse oximetry system of claim 1, wherein the one or more position indicators comprise a first position indicator disposed laterally relative to the emitter and a second position indicator disposed laterally relative to the detector, wherein the first position indicator and the second position indicator are each separated from the respective emitter and detector by known, fixed distances.
 3. The pulse oximetry system of claim 1, wherein the one or more position indicators comprise a first contact point and a second contact point of a linear resistor of a linear potentiometer device.
 4. The pulse oximetry system of claim 3, wherein a digit, when placed within the sensor body, acts as a variable linear resistor between the first contact point and the second contact point.
 5. The pulse oximetry system of claim 1, wherein the one or more position indicators comprise a first contact point and a second contact point of a potentiometer device that measures bending or flex of a substrate based on changes in resistance between the first contact point and the second contact point.
 6. The pulse oximetry system of claim 1, wherein the one or more position indicators comprise one or more Hall Effect devices that generate a voltage in response to movement of a magnet relative to the Hall Effect devices.
 7. A pulse oximetry monitor, comprising: one or more processors configured to: derive relative positions of an emitter and a detector of a pulse oximetry sensor using non-optical data generated by one or more position indicators disposed on a sensor body of the pulse oximetry sensor; determine a path length of light from the emitter and the detector based on the relative positions; and determine a physiological parameter based on a physiological data signal from the sensor and a relationship between light absorbance or light scatter probabilities as a function of the path length.
 8. The pulse oximetry monitor of claim 7, wherein the one or more processors are configured to calculate correction factors or parameters for the physiological data signal acquired using the pulse oximetry sensor based on the derived relative positions.
 9. The pulse oximetry monitor of claim 7, wherein the one or more processors are configured to derive at least a distance between the emitter and the detector based on the derived relative positions.
 10. The pulse oximetry monitor of claim 7, wherein the non-optical data comprises differential resistance or voltage data indicative of the positions of the emitter and the detector relative to one another.
 11. The pulse oximetry monitor of claim 7, wherein the one or more position indicators comprise components of a potentiometer or of a Hall Effect device. 